Variable frequency circuit in attenuator and/or negative voltage generator

ABSTRACT

Apparatus and methods for distributing spurious tones through the frequency domain are disclosed. One such apparatus can include a dithering circuit configured to generate a sequence of numbers that exhibit statistical randomness and a variable frequency circuit configured to adjust a frequency of an output based on the sequence of numbers so as to spread energy of spurious tones in a frequency response of the output to lower a noise floor. In one example, spurious tones can be reduced in a negative voltage generator of a radio frequency (RF) attenuator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/399,910, filed Jul. 24, 2014, titled “VARIABLE FREQUENCY CIRCUITCONTROLLER,” which is a continuation of U.S. Pat. No. 13/427,616, filedMar. 22, 2012, titled “VARIABLE :FREQUENCY CIRCUIT CONTROLLER,” issuedas U.S. Pat. No. 8,823,459, which claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application No. 61/467,501, filedMar. 25, 2011, titled “VARIABLE FREQUENCY CIRCUIT CONTROLLER,” thedisclosures of each of these applications are hereby incorporated byreference herein in their entireties.

BACKGROUND Field

The disclosed technology relates to electronic systems and, inparticular, to variable frequency circuits,

Description of the Related Art

Certain electronic systems, such as base stations, can include anattenuator to reduce an amplitude and/or power of a radio frequency (RF)signal without appreciably distorting features of the RF signal. Someattenuators can include an amplifier, a digital step attenuator (DSA),and a serial peripheral interface (SPI) controller. In someimplementations, the SPI controller can convert a serial control wordinto an output word, which can control switches in the DSA. The bitstream can represent a time series of one or more bits. The output wordcan be represented as differential signals to increase performanceand/or reduce power consumption. A negative voltage level can be used togenerate the differential signals, for example, using a level shifter.The negative voltage can be generated by the SPI controller. Forexample, an oscillator can generate an output having a fixed frequency,and provide the output to a charge pump via a buffer. The charge pumpcan then generate the negative voltage.

Electronic systems that induce attenuators can have stringent noisespecifications that are difficult to meet. Conventional negative voltagegenerators can introduce noise in the form of spurs, which may also bereferred to as “spurious tones,” at harmonics of a frequency of anoscillator. Such noise can be introduced into RF inputs and/or RFoutputs of some electronic systems. This can, for example, reduce noisemargins. Accordingly, a need exists for reducing spurious tones inelectronic systems, such as electronic systems that include attenuators.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

The apparatus and methods described in the claims each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, some prominentfeatures will now be briefly discussed.

One aspect of the disclosure is an apparatus that includes a ditheringcircuit and a variable frequency circuit. The dithering circuit isconfigured to generate a sequence of numbers that exhibit statisticalrandomness. The variable frequency circuit is configured to adjust afrequency of an output based on the sequence of numbers so as to spreadenergy of spurious tones in a frequency response of the output to lowera noise floor.

According to certain implementations, the variable frequency circuit caninclude a variable oscillator configured to adjust the frequency of theoutput based on the sequence of numbers. In some of theseimplementations, the variable oscillator can be configured to adjust thefrequency of the output by selectively electrically coupling one or morecapacitive circuit elements to the output based on the sequence ofnumbers.

In accordance with various implementations, the variable frequencycircuit can include a programmable frequency divider configured toadjust the frequency of the output based on the sequence of numbers.

According to a number of implementations, the dithering circuit caninclude a linear feedback shift register. Alternatively or additionally,a clock signal of the dithering circuit can be based on the output ofthe variable frequency circuit in accordance with certainimplementations. For instance, the output of the variable frequencycircuit can be a clock signal of the dithering circuit.

According to several implementations, the apparatus can also include acharge pump controlled based on the output of the variable frequencycircuit.

The apparatus can be used in a variety of applications. For example, theapparatus can be used in a radio frequency (RF) communication system toreduce noise associated with an RF signal. As another example, theapparatus can be used in a base station to reduce noise. As anotherexample, the apparatus can be used in a mobile device to reduce noiseassociated with at least one of a received signal and a transmittedsignal. As another example, the apparatus can be used in a microwavepoint-to-point communication system to reduce noise. As yet anotherexample, the apparatus can be used in an attenuator module to generate anegative voltage.

The sequence of numbers can be generated randomly or pseudo-randomly bythe dithering circuit.

Another aspect of the disclosure is an integrated circuit that includesa controller and an attenuator component. The controller includes anegative voltage generator. The negative voltage generator has avariable frequency circuit configured to vary a frequency of an outputso as to randomly or pseudo-randomly spread spurious tones through afrequency domain. The negative voltage generator also has a charge pumpconfigured to generate a negative voltage based on the output of thevariable frequency circuit. The controller is configured to generate oneor more control signals. The attenuator component is configured toreceive a radio frequency (RF) input and the differential pair ofcontrol signals and generate an attenuator component output based on thereceived signals.

According to certain implementations, the controller can further includea level shifter configured to receive the negative voltage and generatea differential pair of control signals, in which the one or more controlsignals generated by the controller include the differential pair ofcontrol signals generated by the level shifter.

In some implementations, the integrated circuit can also include anamplifier configured to generate an RF signal based on the attenuatoroutput.

According to certain implementations, the controller can further includecircuitry configured to generate one or more control words based oncontrol data and provide the one or more control words to the levelshifter. Alternatively or additionally, the controller can furtherinclude a dithering circuit configured to generate a pseudo-random bitstream, and the frequency of the output of the variable frequencycircuit can be based on the pseudo-random bit stream. In accordance withvarious implementations, the controller can be a serial peripheralinterface controller.

In a number of implementations, the attenuator component can be adigital step attenuator component.

Another aspect of the disclosure is an electronic system that includesan antenna, a baseband processor configured to process baseband signals,and a transceiver in operative communication with the antenna and thebaseband processor. The transceiver is configured to generate a bitstream that exhibits statistical randomness and vary a frequency of avariable frequency signal based on the bit stream no as to distributespurious tones in a frequency domain of the variable frequency signal.

In some implementations, the transceiver can include a transmit pathconfigured to receive data from the baseband processor, generate thevariable frequency signal, and transmit a radio frequency (RF) signalvia the antenna. Alternatively or additionally, the transceiver caninclude a receive path configured to receive an RF signal from theantenna, generate the variable frequency signal, and provide receivedata to the baseband processor.

In accordance with certain implementations, e electronic system can be abase station.

Another aspect of the disclosure is a method that includes receiving asequence of numbers that exhibit statistical randomness, anddistributing spurious tones through a frequency domain of a signal basedon the sequence of numbers.

In some implementations, the method further includes generating thesequence of numbers. According to a number of implementations,distributing includes adjusting a frequency of the signal based on thesequence of numbers. In accordance with several implementations, themethod further includes generating a negative voltage based on thesignal. In a number of implementations, the signal can be a radiofrequency signal.

Yet another aspect of this disclosure is an apparatus that includesmeans for generating a pseudo-random bit stream, and means for spreadingthe energy of spurious tones in a frequency response of a signal basedon the pseudo-random bit stream.

According to some implementations, the means for spreading energy caninclude a variable oscillator configured to adjust the frequency of thesignal based on the pseudo-random bit stream. In certainimplementations, the means for spreading energy can include aprogrammable frequency divider configured to adjust the frequency of theoutput based on the pseudo-random bit stream. In accordance with variousimplementations, a clock signal of the means for generating thepseudo-random bit stream is based on a signal generated by the means forspreading energy.

For purposes of summarizing the disclosure, certain aspects, advantagesand novel features of the inventions have been described herein. It isto be understood that not necessarily all such advantages may beachieved in accordance with any particular embodiment of the invention.Thus, the invention may be embodied or carried out in a manner thatachieves or optimizes one advantage or group of advantages as taughtherein without necessarily achieving other advantages as may be taughtor suggested herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts an illustrative electronic system thatincludes a radio frequency (RF) attenuator.

FIG. 2 schematically depicts an illustrative RF attenuator.

FIG. 3 schematically depicts an illustrative controller with a negativevoltage generator.

FIG. 4 graphically illustrates ripples on signals in the time domainthat are introduced by a fixed oscillator.

FIG. 5 graphically illustrates a frequency response of a negativevoltage signal generated by the fixed oscillator in an RF frequencyband.

FIG. 6 schematically illustrates a negative voltage generator accordingto an embodiment.

FIG. 7 schematically illustrates a dithering circuit according to anembodiment.

FIG. 8 schematically depicts an illustrative variable oscillator.

FIG. 9 graphically illustrates a frequency response of a variableoscillator according to an embodiment.

FIG. 10 schematically illustrates another embodiment of a negativevoltage generator.

FIG. 11 is a flowchart of an illustrative method of distributingspurious tones in the frequency domain, according to an embodiment.

FIG. 12 is a block diagram of an apparatus according to an embodiment.

FIGS. 13A-13C illustrate example applications for one or more aspects ofthis disclosure.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Generally described, aspects of the present disclosure relate toreducing spurious tones on an output of an oscillator and/or anothercircuit configured to generate a high frequency signal. According toaspects of this disclosure, spurious tones can be reduced in an outputof a variable frequency circuit, which can include, for example, avariable oscillator or a variable frequency divider. In someimplementations, a variable frequency circuit may be part of a negativevoltage generator. The negative voltage generator may be included in anattenuator, for example, in a base station, access point, or a radiofrequency (RF) repeater system. Spurious tones can occur at a harmonicof a frequency of the output of an oscillator in the variable frequencycircuit. The spurious tones can add unwanted noise to a negative voltagethat can be used, for example, to generate differential control signalsfor the attenuator. Noise from the spurious tones can consume part of anoise tolerance, which is stringent in certain applications. In someimplementations, spurious tones can cause the attenuator to fail a noisespecification, which can, in some cases, result in functional failures.

According to the apparatus and methods described herein, noise fromspurious tones of an oscillator can be spread randomly orpseudo-randomly though the frequency domain, thereby lowering a noisefloor of spurious tones of a frequency response of a signal, such as thenegative voltage generator output. This can be achieved, for example,using a variable oscillator controlled by a dithering circuit. Thedithering circuit can generate a sequence of numbers that exhibitstatistical randomness. For instance, the dithering circuit can generatea pseudo-random bit stream of one or more bits, or any suitablepseudo-random voltage, to dynamically adjust frequency of the variableoscillator. A random bit stream can be used in place of thepseudo-random bit stream in accordance with any of the principles andadvantages described herein. In some implementations, the ditheringcircuit can include a linear feedback shift register. The pseudo-randombit stream can be used to selectively electrically couple one or morecapacitive elements in parallel and/or in series with a capacitance ofthe variable oscillator. A frequency of an output of the variableoscillator can be adjusted based on how much capacitance is in paralleland/or in series with the capacitance of the variable oscillator. Thedithering circuit can cause the frequency of the variable oscillator tobe adjusted in a pseudo-random fashion. This can randomize the spurioustones associated with harmonics of an output of the variable oscillatorthrough the frequency domain. In certain implementations, the ditheringcircuit can use an output of the variable oscillator as a clock signal.This can add another layer of randomness to varying the frequency of thevariable oscillator.

The apparatus and methods described herein may be able to achieve one ormore of the following advantageous features, among others. First, anoise floor of signal, such as an output of a negative voltagegenerator, can be reduced. Second, spurious tones in a signal, such as anegative voltage signal, can be reduced with little or no increase inphysical circuit area. Third, noise problems can be addressed at or nearthe source instead of filtering downstream, which can consume additionalarea for filters and/or special pad placement with substrate isolation.Fourth, signal integrity can be maintained without lowering a controlvoltage level. Although this disclosure may describe features in thecontext of a negative voltage generator and/or an attenuator, any of theprincipals and advantages described herein can be applied to anyvariable frequency circuit with a need for reducing spurious tones.

Any of the principles and advantages herein can be applied to a widevariety of applications in which spurious tones can advantageously bereduced, for example, in a base station, an access point, or a RFrepeater system. FIG. 1 schematically depicts an illustrative electronicsystem 10 that includes a radio frequency (RF) attenuator. Theelectronic system 10 can be, for example, a base station.

The electronic system 10 can be used for transmitting and/or receivingRF signals using a variety of communication standards, including, forexample, Global System for Mobile Communications (GSM), Code DivisionMultiple Access (CDMA), Bluetooth, ISA100.11a, IEEE 802.15.4, and/orWireless HART, as well as other proprietary and non-proprietary Gaussianfrequency-shift keying (GFSK) and frequency-shift keying (FSK) basedschemes, including multi-level FSK schemes, such as 4FSK.

The electronic system 10 can include a transceiver component 11configured to generate RF signals for transmission via an antenna 15 a,and to receive incoming RF signals from an antenna 15 a, 15 b. Thetransceiver component 11 can include one or more transmit paths 12 andone or more receive paths 13 a, 13 b. The transceiver component 11 caninclude one or more attenuators 19, which can include negative voltagegenerators.

A receive path 13 a, 13 b can include, for example, one or more of thefollowing components: a low noise amplifier 17, an attenuator 19, anoscillator 21, a first mixer 23 a, a second mixer 23 b, a firstprogrammable gain amplifier 25 a, a second programmable gain amplifier25 b, a first filter 27 a, a second filter 27 b, a firstanalog-to-digital converter 29 a, and a second analog-to-digitalconverter 29 b.

An RF signal can be received at the antenna 15 a, 15 b. The receive path13 a can provide an input voltage to the low noise amplifier 17 via aswitch 31, for example, when the antenna 15 a that is also selectivelycoupled to the transmit path 12 is not used for transmitting one or moresignals. Alternatively or additionally, the receive path 13 b can filterthe RF signal provided by the antenna with an input filter 33. The inputfilter 33 can be a low pass filter, a band pass filter, or a high passfilter, depending on the application.

The low noise amplifier 17 can include an input for receiving an inputvoltage. The input voltage can be an RF signal. For example, theelectronic system 10 can receive a packet from a transmittertransmitting over the RF, UHF, VHF, and/or ISM frequency bands at adistance of up to several kilometers. Thus, the carrier frequency of theinput voltage can be of a variety of values, including, but not limitedto, a carrier frequency ranging between about 400 MHz to about 3 GHz.

The input voltage received by the low noise amplifier 17 can have arelatively small magnitude. The low noise amplifier 17 can be configuredto amplify the signal, while adding a relatively small amount of noise.The low noise amplifier 17 can then provide an output to an attenuator19, which can include a digital step attenuator. The attenuator 19 canreduce the amplitude of the low noise amplifier output withoutappreciably distorting features of the low noise amplifier output. Theattenuator 19 can also provide a variable gain. For example, a controlinput can be used to adjust the gain. An attenuator 19 can be used toreduce the amplitude of a signal before demodulating the signal, in someimplementations. The attenuator 19 can receive a control input form thebaseband processor 35, for example, to adjust the gain. More detailsregarding attenuators will be provided later.

The first and second mixers 23 a, 23 b can be configured to receivefirst and second local oscillator clock signals from the localoscillator 21. The first and second local oscillator clock signals canhave about the same frequency and a phase difference equal toapproximately a quarter of a period, or about 90°. The first and secondmixers 23 a, 23 b can be configured to down convert the output of theattenuator 19 using the first and second local oscillator clock signals,respectively, thereby generating first and second demodulated signals.The first and second demodulated signals can have a relative phasedifference of approximately a quarter of a period, or about 90°. In someembodiments, the first and second demodulated signals can be used in a Qpath and an I path. In one embodiment, one of the first or secondoscillator clock signals is generated by phase shifting from the other.

The first and second local oscillator clock signals can each have afrequency selected so as to achieve a desired intermediate frequencyand/or baseband frequency for the first and second demodulated signals.For example, multiplying the amplified input signal by a sinusoidalsignal from the oscillator can produce a mixed signal having a frequencycontent centered about the sum and difference frequencies of the carrierfrequency of the attenuator output signal and the frequency of the localoscillator 21.

The first demodulated signal can be amplified, for example, with aprogrammable gain amplifier 25 a, 25 b. Then the output of theprogrammable gain amplifier 25 a, 25 b can be filtered with any suitablefilter 27 a, 27 b (for example, a low pass, band pass, or high passfilter). The output of the filter can be provided to ananalog-to-digital converter 29 a, 29 b. The analog-to-digital converter29 a, 29 b can digitize an input to any suitable resolution. Forinstance, the analog-to-digital converter 29 a, 29 b can be a 1-bitanalog-to-digital converter, such as a limiter or a saturatedanalog-to-digital converter, and the output can be a one-bit digitalsignal. The digital to analog converter 29 a, 29 b can convert an analoginput to a digital output and provide the digital output to the basebandprocessor 35 for further processing. In this way, analog signals in thereceive path 13 a, 13 b can be processed by the baseband processor 35.The second demodulated signal can be processed with separate andsubstantially similar components as the first demodulated signal so adigital signal corresponding to the second demodulated signal can alsobe provided to the baseband processor 35.

The baseband processor 35 can be any suitable processor configured toprocess a baseband signal. For instance, the baseband processor 35 caninclude a digital signal processor, a microprocessor, a programmablecore, the like, or any combination thereof. Moreover, in someimplementations, two or more baseband processors 35 can he included inthe electronic system 10.

A transmit path 12 can receive data from the baseband processor 35 andcause the received data to be transmitted as an RF signal via an antenna15 a. In some implementations, a transmit path 12 can transmit the RFsignal via an antenna 15 a also used by a receive path 13 a. In otherimplementations, a transmit path 12 can transmit the RF signal via anantenna that is not shared with a receive path. Similarly, in someimplementations, a receive path 13 b can receive an RF signal fromantenna 15 b that is not shared with a transmit path 12. A transmit path12 can include, for example, one or more of the following components: afirst and second digital-to-analog converters 37 a, 37 b, first andsecond filters 39 a, 39 b, first and second mixers 41 a, 41 b, a localoscillator 43, a combiner 45, an attenuator 19, an output filter 47, anda power amplifier 49.

The baseband processor 35 can output a signal for a Q path and a signalfor an I path. Each of these signals can be processed substantially thesame in parallel with separate hardware until they are combined. Thedigital-to-analog converter 37 a, 37 b can receive a digital signal fromthe baseband processor 35 and convert the digital signal to an analogsignal. The digital-to-analog converter 37 a, 37 b can have any suitableprecision. The output of the digital-to-analog converter 37 a, 37 b canbe filtered via the first or second filters 39 a, 39 b before being upconverted using a mixer 41 a, 41 b clocked by an output of the localoscillator 43. The mixer 41 a, 41 b can convert a baseband input to RF.

Signals from the Q path and the I path can be combined via the combiner45 so that they can be transmitted in a single RF signal. An output ofthe combiner 45 can be provided to the attenuator 19, which can includea digital step attenuator. The attenuator 19 can have a variable gain.For example, the gain of the attenuator 19 can be controlled based onone or more control signals provided by the baseband processor 35. Moredetails regarding attenuators will be provided later, for example, withreference to FIG. 2.

The output of the attenuator 19 can be filtered. For example, a lowpass, band pass, or high pass filter can filter the output of theattenuator 19. This can, for example, remove noise and/or unwantedfrequency bands. An output of the output filter 47 can be amplified by apower amplifier 49. The power amplifier 49 can have two or more stagesin some implementations. The power amplifier 49 can provide an amplifiedRF signal to the antenna 15 a for transmission from the electronicsystem 10. In some implementations, the RF signal is provided to theantenna 15 a via a switch 31.

Referring to FIG. 2, an illustrative attenuator 19 will be described. Asshown in FIG. 2, the attenuator 19 is an RF attenuator module. The REattenuator module can be implemented on a single integrated circuit insome implementations. The RF attenuator module can be implemented on twoor more integrated circuits in other implementations. An integratedcircuit that includes the attenuator 19 can be encased within a packageand have a plurality of pins for receiving various input signals andproviding various output signals. For example, as illustrated in FIG. 2,the integrated circuit including the attenuator 19 can have 32 leads forreceiving control signal(s), data signal(s), ground, power, or anycombination thereof. The pins can receive analog and/or digital inputsignals. Further, some pins can be specifically configured to receive RFsignals. In addition, the pins can provide output signals, such as RFsignals, for use with other integrated circuits.

The attenuator 19 can include one or more digital step attenuator (DSA)components 52 a, 52 b, one or more RF amplifiers 54 a, 54 b, and acontroller 56, which can be a serial peripheral interface (SPI)controller is some implementations. A digital step attenuator component52 a, 52 b can reduce the amplitude of an RF input signal RFIN_1, RFIN_2without appreciably distorting the features of the RF input signal. Thedigital step attenuator component 52 a, 52 b can be controlled by thecontroller 56, such as an SPI controller, to be enabled/disabled and/orto adjust a gain of the RF input signal. The output of the digital stepattenuator component 52 a, 52 b can be amplified by an amplifier 54 a,54 b. The amplifier 54 a, 54 b can be any suitable amplifier configuredto amplify an RF signal. The amplifier 54 a, 54 b can have any suitablegain for a desired application. The gain can be controlled, for example,by a bias voltage AMP1_BIAS, AMP2_BIAS and/or a supply voltage AMP1_VCC,AMP2_VCC provided to the amplifier 54 a, 54 b. The output of theamplifier can be provided as an RF output signal RFOUT_1, RFOUT_2 of theattenuator 19. The RF output signal can be used in a transmit path orreceive path, for example, as described earlier with reference to FIG.1.

The illustrated controller 56 can convert a serial control word into oneor more output words that can control switches, or other circuitelements, in the DSA component 52 a, 52 b. For example, the controller56 can generate a pair of parallel words of any suitable number of bits(for example, 6 bits per word) in certain implementations. in thisexample, one word can control the first DSA component 52 a and the otherword can control the DSA component 52 b. Each bit in each control wordcan be represented as a complementary signal. A complementary pair ofsignals can include a first signal with a logic value of “1” (or “0”)and a second signal with a complementary logic value of “0” (or “1”).The logic “1” may be implemented as a positive voltage while the logic“0” may be implemented as a zero voltage (for example, ground). Such apair of complementary signals can be used for controlling a stage ofattenuation in a DSA component, because such control can include turningon one transistor in the DSA component while simultaneously turning offanother transistor in the DSA component. Further, the USA componentperformance can be improved by representing the logic “0” signal with anegative voltage instead of a zero voltage. In this case, thecomplementary pair of control signals becomes a differential pair ofcontrol signals. The DSA component performance can be improved with thedifferential control signals, for example, because the transistor in theDSA component that is turned off is forced further into its off regionby the negative voltage. The controller 56 can generate the negativevoltage, for example, using level shifters to convert one or moresingle-ended control lines into one or more differential pairs.

Referring to FIG. 3, more detail will be provided about an illustrativecontroller 56. The controller 56 can include a SPI control circuitry 58,a negative voltage generator 60, and a level shifter 62. The SPI controlcircuitry 58 can receive SPI data SPI_DATA, a clock SPI_CLK, a latchenable SPI_LE, and a reset signal RST. The illustrated SPI controlcircuitry 58 can generate two output words PO1[5:0] and PO2[5:0] from aserial bit stream of SPI data SPI_DATA. The output words can be providedto the level shifter 62.

The negative voltage generator 60 can generate a negative voltage NV andprovide the negative voltage NV to the level shifter 62. The levelshifter 62 can use the negative voltage NV to generate differentialcontrol signals corresponding to the two output words received from theSPI control circuitry 58. The differential control signals can controlan attenuator component, such as the DS A component 52 a, 52 b. Usingdifferential control signals can increase performance and/or reducepower consumption.

The negative voltage generator 60 can include an oscillator and a chargepump to control the negative voltage NV and/or switching time. Theoverall system, or any subsystem thereof, may have stringent noisespecifications. However, the circuit topology of some conventionalnegative voltage generators can introduce unwanted noise and/or spurioustones into the system. For example, negative voltage generators with afixed oscillator frequency have created spurious tones on the negativevoltage generated. A spurious tone or plurality of spurious tones canrepresent an unwanted noise component of a signal.

FIG. 4 illustrates tipples in the time domain that correspond tospurious tones in the frequency domain. FIG. 4 graphically illustratesripples from a negative voltage generator with fixed oscillator in thetime domain. The recurring ripples on the graph of voltage over time inFIG. 4 can produce the unwanted spurious tones in the frequency domain,as will be described later. These ripples can result from coupling of afixed oscillator that generates a signal of a fixed frequency. The topwaveform illustrated in FIG. 4 shows ripples on a negative voltagesignal at the output of the negative voltage generator. These ripplesare then coupled to the differential output signals of a level shifter.Ripples on the level shifter output are shown in the bottom waveform ofFIG. 4. These ripples can further couple onto the RF signal that passesthrough the DSA component, resulting in unwanted noise components on anRF output signal.

The ripples shown in FIG. 4 occur in time, at a rate that can beapproximately equal to a fixed frequency used to generate the negativevoltage. FIG. 5 graphically illustrates the same negative voltage signalin the frequency domain, in which a large number of harmonics can beseen. These harmonics are multiples of the fixed frequency used togenerate the negative voltage. The order of these harmonics can extendrelatively high, for example, into the hundreds. As a result, thefrequency of these harmonics can reach as high as a few GHz or more.These harmonics can extend into the RF band, overlapping the samefrequency region as the RF signal that is passed through the DSAcomponent. When these harmonics couple from the level shifter to the DSAcomponent, they can add to the RF signal passed through the DSAcomponent, appearing as added noise. In some applications, noise on anRF signal ranging from about −130 dB to −140 dB is considered to bewithin an acceptable range. However, as shown in FIG. 5, the noise isabove -130 d B for substantially the entire RF frequency band. Thus, thecircuit corresponding to FIG. 5 does not have a comfortable noisemargin. In some instances, this may lead to failing system noisespecifications.

As shown in FIGS. 4 and 5, spurious tones on the negative voltage canpropagate through to an RF output signal. For example, spurious tones onthe negative voltage NV can couple to the differential output words ofthe level shifter 62. The spurious tones on the differential outputwords can also introduce unwanted noise through one or more switches ofthe digital step attenuator component 52 a, 52 b to the RF outputRFOUT_1 and/or RFOUT_2 of the attenuator 19. As a result, overall noiseperformance can be degraded. The spurious tones can reduce a dynamicrange, reduce a signal to noise ratio, reduce a noise margin, the like,or any combination thereof. These effects can increase the likelihood offunctional errors, increase sensitivity to noise on input signals,degrade RF output signals and/or degrade RF signals received from thesystem, the like, or any combination thereof. In some instances,spurious tones introduced by a fixed oscillator in a negative voltagegenerator can cause a system to fail a noise figure specification.

FIG. 6 schematically illustrates a negative voltage generator 60 aaccording to an embodiment. The negative voltage generator 60 a can beimplemented in any part with a need for a negative voltage. For example,any negative voltage generator described herein can be implemented inthe attenuator 19, other variable attenuators in a receive chain of aReceive Tower Top Amplifier (MA), or in a bias controller for RFswitches which use D-Mode pHEMT field effect transistors. The negativevoltage generator 60 a can implement a dynamic approach to spreadingnoise energy from a variable frequency circuit, such as a variableoscillator, so as to lower a noise floor of spurious tones on harmonicsof the negative voltage ripples. The negative voltage generator 60 a canspread noise energy by controlling a variable frequency circuit 61 witha dithering circuit 62. In the negative voltage generator 60 aillustrated in FIG. 6, the variable frequency circuit 61 includes avariable oscillator 64. In addition to the variable frequency circuit 61and the dithering circuit 62, the negative voltage generator 60 a caninclude a buffer 66 and a charge pump 68.

The dithering circuit 62 can generate a pseudo-random number in the formof a bit stream based on an output Q of the variable frequency circuit61. The pseudo-random bit stream may be provided as an output of thedithering circuit 62. The bit stream generated by the dithering circuit62 can be referred to as pseudo-random because the bit stream may appearrandom, although the bit stream is generated by a deterministic causalprocess. The pseudo-random bit stream exhibits statistical randomness.Yet, the pseudo-random bit stream generated by the dithering circuit 62can be represented by a logic function.

As shown in FIG. 6, the output Q of the variable frequency circuit 61can be buffered by a buffer 66 and then a buffered output Qp can beprovided to the dithering circuit 62. In some implementations, a clocksignal of the dithering circuit 62 can be based on the output of thevariable frequency circuit 61. This can add a second level of randomnessto the pseudo-random bit stream generated by the dithering circuit 62because a frequency of the clock should vary as a frequency of theoutput Q of the variable frequency circuit 61 is adjusted.

Referring to FIG. 7, an illustrative dithering circuit 62 will bedescribed. The dithering circuit 62 can be implemented using anysuitable combination of analog and digital circuit elements. In someimplementations, the dithering circuit 62 can be implemented usingdigital logic. The dithering circuit 62 can include a linear feedbackshift register (LFSR). The length of the LFSR in bits can be selected,for example, based on a desired level of randomness in the output of thedithering circuit 62. The LFSR can include a plurality of state elements70, such as flip-flops, latches, the like, or any combination thereof.One state element 70 can implement each bit of the LFSR. For example, asillustrated in FIG. 7, the dithering circuit 62 includes a 16-bit LFSRand 16 flip-flops are included.

To vary the bit stream generated by the LFSR, logic circuits can beincluded between one or more state elements. The logic circuits canreceive outputs from two or more state elements and implement anysuitable logic function (for example, NAND, NOR, NOT, XOR, the like, orany combination thereof). For example, in the illustrated. ditheringcircuit 62, XOR gates 72 a, 72 b, 72 c are implemented between some ofthe state elements 70. More specifically, in the illustrated ditheringcircuit 62, XOR gate 72 a is implemented between the 4^(th) and 5^(th)state elements 70-4 and 70-5, respectively, XOR gate 72 b is implementedbetween the 14^(th) and 15^(th) state elements 70-14 and 70-15,respectively, and XOR gate 72 c is implemented between the 15^(th) and16^(th) state elements 70-15 and 70-16, respectively. XOR gates may beused for one or more logic circuits between state elements for themathematical properties of the XOR logic function. In someimplementations, different logic circuits can be included betweendifferent state elements. In other implementations, the same logiccircuits (for example, XOR gates) can be implemented between stateelements, for example, as shown in FIG. 7, In any of theseimplementations, some state elements may provide an output to the inputof the next state element in the LFSR without passing through a logiccircuit. For example, there are no logic circuits implemented betweenthe 5^(th) to 14^(th) state elements 70-5 to 70-14, respectively, andthe 1^(st) to 4^(th) state elements 70-1 to 70-4, respectively. In thisexample, the Q output of one state element can be provided to the Dinput of an adjacent state element.

The length in bits of the LFSR and/or the placement of XOR gates, whichcan be referred to as a “feedback path polynomial,” can impact thepseudo-random sequence of bits generated by the dithering circuit 62. Asa result, the length of the LFSR and/or the placement of XOR gates canalso impact how the variable oscillator is controlled. Adjusting thefrequency of the variable oscillator 64 (for example, via dynamicswitching) can affect a spur level and resulting harmonics in thefrequency spectrum of an output of the variable oscillator 64 and/orother signals in the data path, such as the negative voltage signal andthe RF output of the DSA 19. In some implementations, the LFSR lengthand/or the feedback path polynomial can be programmable to tune and/oroptimize the spreading of unwanted spurious tones. This can allow theLFSR and/or the feedback path polynomial to be adjusted based on adesired frequency range of operation or other parameters.

The logic circuits can be configured to receive one or more inputs fromany of the state elements in the LFSR. For example, in the illustrateddithering circuit 62, the logic circuits receive each one input from theoutput of one state element (i.e., the 1^(st) state element 70-1) andanother input from a different state element (i.e., the 16^(th),15^(th), or 5^(th) state elements 70-16, 70-15, or 70-5, respectively).In this way, the bit stream generated by the illustrated ditheringcircuit 62 can be pseudo-random.

In certain implementations, the clock signal CLK can be a bufferedversion of an output from the variable frequency circuit 61 (forexample, an output of the variable oscillator 64). Because the output ofthe variable frequency circuit 61 changes in frequency, the frequency ofthe clock signal CLK an also change. With an adjusted frequency, theclock signal CLK can add additional randomness to the bit streamgenerated by the dithering circuit. More specifically, the data storedby one or more state elements can be different based on the clock signalCLK capturing data at different points in time due to the adjustedfrequency.

The pseudo-random bit stream generated by the dithering circuit 62 cancontrol the frequency of the variable frequency circuit 61. For example,when the variable frequency circuit 61 includes a variable oscillator64, the pseudo-random bit stream can be used to adjust the frequency thevariable oscillator 64.

The pseudo-random bit stream can include any suitable number of bits.For example, the pseudo-random bit stream can range from about 1 to 16bits. For example, as illustrated in FIGS. 6 and 7, the pseudo-randombit stream can be four bits. The number of bits can depend on the numberof bits used to control the variable frequency circuit 61 and/or adesired precision of a frequency of an output of the variable frequencycircuit 61.

In addition to generating the pseudo-random bit stream, the ditheringcircuit 62 can tune the output bit stream with the two control wordsFRC_VAL[3:0] and FRC_CTRL[3:0], according to certain implementations.These control words can control a number of bits and/or a range offrequencies over which dithering occurs. For instance, a fixed frequency(for example, a lower frequency) can be selected using the controlwords. A first control word can provide an output word FRC_VAL[3:0] thatcan output instead of the pseudo-random bit stream. The first controlword can be used to select the frequency of the output word. A secondcontrol word FRC_CTRL,[3:0] can be used to select whether to provide thepseudo-random bit stream or the first control word as the output of thedithering circuit 62. For example, the second control word can be aselect input to a multiplexer configured to select between thepseudo-random bit stream or the first control word. In someimplementations, the second control word can be one bit to select eitherthe pseudo-random bit stream or the control word. In otherimplementations, the second control word can select any number of bits(for example, each bit can be selected) between outputting bits of thepseudo-random bit stream or bits of the first control word. Any portionof the first control word and/or the second control word can becontrolled via software and/or hardware.

Referring back to FIG. 6, the output of the dithering circuit 62 cancontrol the frequency of the variable frequency circuit 61. The variablefrequency circuit 61 can be any circuit configured to generate an outputhaving a frequency that is adjustable based on the output of thedithering circuit. In some implementations, the variable frequencycircuit 61 can include a variable oscillator 64.

Referring to FIG. 8, an illustrative variable oscillator 64 will bedescribed. The output word from the dithering circuit 62 can be used toselectively couple one or more capacitive circuit elements in seriesand/or parallel with a capacitance of the variable oscillator 64. Asillustrated in FIG. 8, the variable oscillator 64 can include aplurality of capacitors in parallel with a capacitance of the variableoscillator 64. The plurality of capacitors can have any suitablecapacitance for generating an output with a desired frequency. In someimplementations, the capacitance of each capacitor can be a multiple ofanother capacitor. For example, as illustrated in FIG. 8, each capacitorhas a capacitance that is twice the capacitance of another capacitor.Such capacitance values can allow each capacitor to adjust the frequencyby a different amount. Each of the capacitors can be electricallycoupled/decoupled from the capacitance of the variable oscillator 64 viaclosing/opening a switch. The switch can be any suitable voltagecontrolled switch, for example, a transistor. Each switch can becontrolled by one bit of the output from the dithering circuit 62.

Adding additional capacitors in parallel can increase the capacitance.With more capacitance, a resonant frequency of the variable oscillatorcan decrease. Conversely, with less capacitance, the resonant frequencyof the variable oscillator can increase. Thus, the output frequency ofthe variable oscillator 64 can be adjusted based on which switches areclosed and/or opened. For instance, the capacitor with a capacitance of8C can adjust the output frequency of the variable oscillator 64 morethan the capacitor with a capacitance of C. In some implementations, thevariable oscillator may include an LC tank.

FIG. 9 graphically illustrates noise in a frequency response of an RFsignal generated using a variable oscillator 64 dynamically controlledby a dithering circuit 62. As illustrated in FIG. 9, simulation resultsshow that controlling the frequency of the variable oscillator 64 with apseudo-random bit stream generated by the dithering circuit 62 cansplatter and/or spread energy of the spurious tones on the RF signal.Unlike the frequency response in FIG. 5, lobes are not pronouncedbecause the spurious tones are randomized through the frequency domain.In addition, the frequency response of FIG. 9 shows a lower noise floor.The noise of the frequency response illustrated in FIG. 9 is lowercompared to the frequency response shown in FIG. 5. For example, noiseis under −130 dB for a large portion of the RF frequency band.

Referring back to FIG. 6, the output Q of the variable frequency circuit61 can be buffered by the buffer 66 and provided to the charge pump 68.In some implementations, the buffer 66 can provide the charge pump 68with differential signals Qp and Qn. The charge pump 68 can generate alower or higher voltage power source via direct current (DC) to DCvoltage conversion. The charge pump 68 can generate a negative voltagebased on the output of the variable frequency circuit 61. The output ofthe variable frequency circuit 61 can control a voltage level and/orswitching time in the charge pump 68. The charge pump 68 can use one ormore capacitors to store charge and one or more capacitors to holdcharge and filter output voltage. This can create either a higher orlower voltage output. One or more switching elements can controlconnections to the storage capacitor(s). To generate a higher voltage,charge can be stored in a storage capacitor by applying a positivevoltage across the storage capacitor. Then the storage capacitor can bedisconnected from the positive voltage and reconnected with a negativeterminal being connected to the positive voltage. This can roughlydouble a voltage across the storage capacitor. These principles can beapplied to generate a negative voltage with the voltage generator 60 a.

Referring to FIG. 10, another illustrative voltage generator 60 b willbe described. The voltage generator 60 b is an alternativeimplementation of the voltage generator 60 a, which can also be used inany application with a need for a negative voltage. The voltagegenerator 60 b can include a dithering circuit 62 and a charge pump 68with any combination of features described herein. However, the variablefrequency circuit 61 is different in the negative voltage generator 60 bcompared to the negative voltage generator 60 a. The variable frequencycircuit 61 of the negative voltage generator 60 b can include a fixedoscillator 90, a buffer 66, and a programmable divider 92. The fixedoscillator 90 can generate an output with a fixed frequency. The fixedfrequency output can be provided to the programmable divider 92. Theprogrammable divider 92 can divide the frequency of the fixed oscillatorbased on the output of the dithering circuit 62. In this way, theprogrammable divider 92 can vary the frequency of one or more outputsignals, which can control the charge pump 68. The one or more outputsignals can be buffered by the buffer 66. The buffer 66 can generatedifferential signals Qp and Qn that can control the charge pump 68 basedon the one or more output signals of the programmable divider 92.Accordingly, the pseudo-random bit stream generated by the ditheringcircuit 62 can cause the programmable divider 92 to randomize spurioustones through the frequency domain and lower a harmonic noise floor.

FIG. 11 is a flowchart of an illustrative method 100 of distributingspurious tones in the frequency domain, according to an embodiment. Anycombination of the features of the method 100 may he embodied in anon-transitory computer readable medium and stored in memory. Whenexecuted, the non-transitory computer readable medium may cause some orall of the method 100 to be performed. It will be understood that themethod 100 may include greater or fewer operations and the operationsmay be performed in any order, as appropriate.

The method 100 can distribute spurious tones through the frequencydomain. This can reduce spurious tones at certain harmonics by spreadingenergy of the spurious tones through the frequency domain of a signal. Asequence of numbers exhibiting statistical randomness can be received atblock 102. The sequence of numbers can be generated randomly orpseudo-randomly. Using the sequence of numbers, a frequency of avariable frequency circuit can be adjusted at block 104. By adjustingthe frequency, spurious tones introduced by the variable frequencycircuit (for example, an oscillator) can be distributed though thefrequency domain of the signal at block 106. In this way, unwanted noisefrom spurious tones created by the by an oscillator can be reduced.According to certain implementations, the signal can be an RF signal. Insome implementations, a negative voltage can be generated based on thesignal. However, it will be understood that method 100 can beimplemented in a variety of contexts.

Any combination of the principles and advantages described herein can beimplemented in a variety of applications For instance, the variablefrequency circuits 61 and the dithering circuits 62 described herein canbe implemented in a wide variety of apparatus. FIG. 12 is a blockdiagram of an apparatus 110 according to an embodiment. As illustrated,the apparatus 110 includes a variable frequency circuit 61 and adithering circuit 62. The dithering circuit 62 can generate a sequenceof numbers that exhibit statistical randomness. The dithering circuit 62can also implement any combination of features of the dithering circuitsdescribed herein. The variable frequency circuit 61 can adjust afrequency of an output based on the sequence of numbers generated by thedithering circuit 62 so as to spread energy of spurious tones in afrequency response of the output to lower a noise floor. The variablefrequency circuit 61 can also implement any combination of features ofthe variable frequency circuits described herein.

FIGS. 13A-13C illustrate example applications for one or more aspects ofthis disclosure. Generally described, aspects of this disclosure can beimplemented in communications systems and, in particular, communicationssystems that include high isolation switches. For example, negativevoltages generated as described herein can be used to slam a highisolation switch in the off position. It will be understood that aspectsof this disclosure can be implemented in transmitters, receivers,transceivers, or any combination thereof. In such applications,apparatus, methods, and integrated circuits described herein can beimplemented to reduce noise and, in some instances, noise associatedwith RF signals that are transmitted and/or received. FIGS. 13A-13Cdepict some illustrative apparatus 110 in communication systemsincluding communications devices 110 a and 110 b, a base station 110 c,a mobile device 110 d, and microwave point-to-point communicationsdevices 110 e and 110 f.

As shown in FIG. 13A, an illustrative communication system 120 caninclude communications devices 110 a and 110 b that communicate witheach other via antennas. In some implementations, the communicationsystem 120 can be an RF communication system. The communication system120 can be configured to communicate using, for example, Wi-Fi (IEEE802.11), Bluetooth, Global System for Mobile (GSM), code divisionmultiple access (CDMA), 3G, 4G, long term evolution (LIE), the like, orany combination thereof. The communications devices 110 a and/or 110 bcan include the variable frequency circuit 61, the dithering circuit 62,the electronic system 10 of FIG. 1, or any combination thereof. Thevariable frequency circuit 61 and the dithering circuit 62 can reducenoise associated with signals, such as RF signals, in the communicationsdevices 110 a and 110 b.

FIG. 13B illustrates a communication system 120 a that includes a basestation 110 c in communication with a mobile device 110 d. The basestation 110 c can implement any combination of features of the ditheringcircuit 62 and/or the variable frequency circuit 62 described herein. Asmentioned above, the electronic system 10 can be embodied in a basestation 110 c, For instance, the dithering circuit 62 and the variablefrequency circuit 61 can be implemented in the base station 110 c toreduce noise. Alternatively or additionally, the mobile device 110 d caninclude any combination of features of the electronic system 10described herein, such as the dithering circuit 62 and the variablefrequency circuit 61. For instance, the dithering circuit 62 and thevariable frequency circuit 61 can be implemented in the mobile device110 d to reduce noise associated with at least one of a received signaland a transmitted signal. In some implementations, the dithering circuit62 and/or the variable frequency circuit 61 can be embodied in a poweramplifier module or other front end module of the mobile device 110 d.Some example mobile devices 110 d include, but are not limited to, acellular phone (for example, a smart phone), a laptop, a tabletcomputer, a personal digital assistant (PDA), an electronic book reader,and a portable digital media player.

FIG. 13C illustrates a microwave point-to-point communication system 110b that can include microwave communications devices 110 e and 110 f Themicrowave communications devices 110 e and 110 f can implement anysuitable transmit and/or receive functionality associated with microwavecommunications. The microwave communications devices 110 e and/or 110 fcan implement any combination of features of the dithering circuit 62and/or the variable frequency circuit 61 described herein. In themicrowave point-to-point communication system 120 b, the ditheringcircuit 62 and the variable frequency circuit 61 can reduce noise.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The words “coupled” or connected”, asgenerally used herein, refer to two or more elements that may be eitherdirectly connected, or connected by way of one or more intermediateelements. Additionally, the words “herein,” “above,” “below,” and wordsof similar import, when used in this application, shall refer to thisapplication as a whole and not to any particular portions of thisapplication. Where the context permits, words in the above DetailedDescription using the singular or plural number may also include theplural or singular number respectively. The word “or” in reference to alist of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list, all of theitems in the list, and any combination of the items in the

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of certain embodiments is not intended tobe exhaustive or to limit the invention to the precise form disclosedabove. While specific embodiments of, and examples for, the inventionare described above for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. For example, while processesor blocks are presented in a given order, alternative embodiments mayperform routines having steps, or employ systems having blocks, in adifferent order, and some processes or blocks may be deleted, moved,added, subdivided, combined, and/or modified. Each of these processes orblocks may be implemented in a variety of different ways. Also, whileprocesses or blocks are at times shown as being performed in series,these processes or blocks may instead be performed in parallel, or maybe performed at different times.

The teachings of the disclosure provided herein can be applied to othersystems, not necessarily the systems described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

1. (canceled)
 2. A radio frequency attenuator comprising: a controllerincluding a variable frequency circuit configured to spread energy ofspurious tones through a frequency domain of an output, the controllerfurther including a buffer configured to receive the output of thevariable frequency circuit and to generate differential signals, and thecontroller configured to generate a control signal based on thedifferential signals; and an attenuator component in communication withthe controller, the attenuator component configured to attenuate a radiofrequency signal based on the control signal.
 3. The radio frequencyattenuator of claim 2 wherein the controller indudes a dithering circuitconfigured to provide an output sequence that exhibits statisticalrandomness to the variable frequency circuit.
 4. The radio frequencyattenuator of claim 3 wherein the controller includes a charge pumpconfigured to receive the differential signals from the buffer.
 5. Theradio frequency attenuator of claim 4 wherein an output of the chargepump is a negative voltage that is maintained below zero Volts.
 6. Theradio frequency attenuator of claim 3 wherein the dithering circuit isconfigured to use a signal of the differential signals as a clocksignal.
 7. The radio frequency attenuator of claim 2 wherein the bufferis configured to receive the output of the variable frequency circuit asa single-ended input.
 8. The radio frequency attenuator of claim 7wherein the controller includes a charge pump configured to receive thedifferential signals from the buffer, and the control signal is based onan output of the charge pump.
 9. The radio frequency attenuator of claim8 wherein the controller includes a dithering circuit configured toprovide an input sequence to the variable frequency circuit.
 10. Theradio frequency attenuator of claim 2 wherein the controller includes alevel shifter configured to generate the control signal.
 11. The radiofrequency attenuator of claim 2 wherein the attenuator component is adigital step attenuator component.
 12. The radio frequency attenuator ofclaim 2 wherein the a enua or component and the controller areimplemented on a single integrated circuit.
 13. A negative voltagegenerator comprising: a variable frequency circuit configured togenerate an output having energy of spurious tones spread through afrequency domain; a buffer configured to receive the output of thevariable frequency circuit as a single-ended input and to generatedifferential signals; and a charge pump configured to generate anegative voltage based on the differential signals generated by thebuffer.
 14. The negative voltage generator of claim 13 furthercomprising a dithering circuit configured to provide an input sequencethat exhibits statistical randomness to the variable frequency circuit.15. The negative voltage generator of claim 14 wherein a clock signal oft dithering circuit is provided by the buffer.
 16. The negative voltagegenerator of claim 15 wherein the variable frequency circuit includes avariable oscillator.
 17. The negative voltage generator of claim 14wherein the variable frequency circuit includes an oscillator, and aclock signal of the dithering circuit is based on an output of theoscillator.
 18. An electronically-implemented method of generating anegative voltage with reduced spurious tones, the method comprising:distributing spurious tones through a frequency domain of a variablefrequency signal; buffering the variable frequency signal to providedifferential signals; and generating the negative voltage with reducedspurious tones based on the differential signals.
 19. Theelectronically-implemented method of claim 18 further comprisingproviding an input sequence that exhibits statistical randomness to avariable frequency circuit configured to generate the variable frequencysignal.
 20. The electronically-implemented method of claim 19 whereinthe providing is performed by a dithering circuit configured to use asignal of the differential signals as a clock signal.
 21. Theelectronically-implemented method of claim 18 wherein the generating isperformed using a charge pump that is in communication with a variablefrequency circuit by way of a buffer arranged to perform the buffering.